Display device and light guide device

ABSTRACT

A display device includes a first diffraction element configured to diffract image light and a second diffraction element disposed on an optical path between a light source and the first diffraction element. The first and the second diffraction element emit light beams that have highest diffraction efficiencies in one direction when a single wave light enters from directions of normals. If the sum of the number of reflections of light and the number of times of intermediate image generation between the second diffraction element and the first diffraction element is an even number, in the first and the second diffraction element, the directions of the diffraction light having the highest diffraction efficiencies are output when light enters from the directions of the normals are the same with respect to the directions of the normals to the incident planes. If the sum is an odd number, the directions are opposite.

BACKGROUND

1. Technical Field

The present disclosure relates to a display device and a light guidedevice.

2. Related Art

Devices that use diffraction elements such as holographic elementsinclude, for example, hologram recording and playback devices, anddisplay devices that guide image light by using diffraction elements soas to enter the eyes of a user. In holographic elements, the pitches ofinterference patterns are optimized so as to obtain the highestdiffraction efficiencies at predetermined wavelengths. As a lightsource, for example, a semiconductor laser may be used. However, even ifa semiconductor laser operates in a single mode, the laser beam has acertain spectrum width, and light beams of wavelengths other than apredetermined wavelength may affect the resolution. Furthermore,wavelengths of light emitted from a semiconductor laser may vary due toambient temperatures and other factors, and such variations may alsoaffect the resolution. To solve the problem, for example,JP-A-2000-338847 proposes a structure in which a holographic elementthat absorbs wavelengths other than a predetermined wavelength isprovided in a prior stage of a diffraction element such as a holographicelement.

If optical components such as a mirror and a lens are disposed betweentwo diffraction elements to reduce the size of the device, increase thedegree of freedom of design, and correct various aberrations, dependingon the directions of the two diffraction elements, beams of light havingwavelengths other than a predetermined wavelength may become incident onpositions notably beyond the original target point, and such deviationmay affect the resolution.

SUMMARY

An advantage of some aspects of the embodiment is that a display deviceand a light guide device capable of reducing a decrease in resolutionand the like due to variations in wavelengths of light emitted from alight source are provided.

To solve the above-mentioned problem, a display device according to anaspect of the embodiment includes an image light generation deviceconfigured to output an image light, a first diffraction elementconfigured to diffract the image light that has entered a first incidentplane such that the image light is directed to an eye of an observer anda second diffraction element disposed on an optical path between theimage light generation device and the first diffraction element, thesecond diffraction element being configured to diffract the image lightthat has entered a second incident plane such that the image light isdirected to the first diffraction element. The first diffraction elementhas highest diffraction efficiency in a first direction when lightenters from a direction of a normal to the first incident plane, thesecond diffraction element has highest diffraction efficiency in asecond direction when light enters from a direction of a normal to thesecond incident plane, the first diffraction element and the seconddiffraction element are disposed such that, if the sum of the number ofreflections of light and the number of times of intermediate imagegeneration between the second diffraction element and the firstdiffraction element is an even number, when viewed from direction ofnormal to virtual plane that include a normal to the first incidentplane and a normal to the second incident plane, the direction of thefirst direction with respect to the direction of the normal to the firstincident plane and the direction of the second direction with respect tothe direction of the normal to the second incident plane are the same aseach other, and the first diffraction element and the second diffractionelement are disposed such that, if the sum of the number of reflectionsof light and the number of times of intermediate image generationbetween the second diffraction element and the first diffraction elementis an odd number, when viewed from the direction of the normal to thevirtual plane that include the normal to the first incident plane andthe normal to the second incident plane, the direction of the firstdirection with respect to the direction of the normal to the firstincident plane and the direction of the second direction with respect tothe direction of the normal to the second incident plane are differentfrom each other.

A light guide device according to an aspect of the embodiment includes afirst diffraction element configured to diffract light that has beenemitted from a light source and that has entered a first incident plane,and a second diffraction element disposed on an optical path between thelight source and the first diffraction element, the second diffractionelement being configured to diffract the light that has entered a secondincident plane such that the image light is directed to the firstdiffraction element. The first diffraction element has highestdiffraction efficiency in a first direction when light enters from adirection of a normal to the first incident plane, the seconddiffraction element has highest diffraction efficiency in a seconddirection when light enters from a direction of a normal to the secondincident plane, the first diffraction element and the second diffractionelement are disposed such that, if the sum of the number of reflectionsof light and the number of times of intermediate image generationbetween the second diffraction element and the first diffraction elementis an even number, when viewed from direction of normal to virtual planethat include a normal to the first incident plane and a normal to thesecond incident plane, the direction of the first direction with respectto the direction of the normal to the first incident plane and thedirection of the second direction with respect to the direction of thenormal to the second incident plane are the same as each other, and thefirst diffraction element and the second diffraction element aredisposed such that, if the sum of the number of reflections of light andthe number of times of intermediate image generation between the seconddiffraction element and the first diffraction element is an odd number,when viewed from the direction of the normal to the virtual plane thatinclude the normal to the first incident plane and the normal to thesecond incident plane, the direction of the first direction with respectto the direction of the normal to the first incident plane and thedirection of the second direction with respect to the direction of thenormal to the second incident plane are different from each other.

According to some aspects of the embodiment, a first diffraction elementis configured to diffract image light emitted from an image lightgeneration device to enter an eye of an observer, and a seconddiffraction element is disposed on an optical path between a lightsource of the image light generation device and the first diffractionelement to absorb wavelengths other than a predetermined wavelength. Thefirst diffraction element and the second diffraction element may be aholographic element or a blazed holographic diffraction element, and thefirst diffraction element and the second diffraction element each outputdiffracted light of highest diffraction efficiency in one direction whena light beam enters from a direction of a normal. With this structure,improper alignment of the first diffraction element and the seconddiffraction element causes a failure in absorption of wavelengths otherthan a predetermined wavelength and results in a decrease in resolutiondue to the variations in the wavelengths. According to some aspects ofthe embodiment, if the sum of the number of reflections of light and thenumber of times of intermediate image generation between the seconddiffraction element and the first diffraction element is an even number,the first diffraction element and the second diffraction element aredisposed such that, when light enters from a direction of a normal tothe first diffraction element and a direction of a normal to the seconddiffraction element, the sides toward which diffraction light beams thathave the highest diffraction efficiencies are output are the same withrespect to the directions of the normals to the incident planes.Accordingly, variations in the wavelengths of the light emitted from alight source can be canceled between the first diffraction element andthe second diffraction element. In contrast, if the sum of the number ofreflections of light and the number of times of intermediate imagegeneration between the second diffraction element and the firstdiffraction element is an odd number, the first diffraction element andthe second diffraction element are disposed such that, when light entersfrom a direction of a normal to the first diffraction element and adirection of a normal to the second diffraction element, the sidestoward which diffraction light beams that have the highest diffractionefficiencies are output are opposite with respect to the directions ofthe normals to the incident planes. Accordingly, variations in thewavelengths of the light emitted from a light source can be canceledbetween the first diffraction element and the second diffractionelement. Consequently, a decrease in resolution or the like due tovariations in the wavelengths of light emitted from a light source canbe reduced.

According to an aspect of the embodiment, the first diffraction elementand the second diffraction element may be reflective holographicelements. According to an aspect of the embodiment, the firstdiffraction element and the second diffraction element may be reflectivevolume holographic elements.

According to an aspect of the embodiment, the first diffraction elementand the second diffraction element may have a plurality of interferencefringes linearly extending parallel to each other. With this structure,plane waves can be used.

According to an aspect of the embodiment, the first diffraction elementand the second diffraction element may have a plurality of curvedinterference fringes extending parallel to each other. With thisstructure, spherical waves can be used.

According to an aspect of the embodiment, the first diffraction elementand the second diffraction element may have a plurality of kinds ofinterference fringes of different pitches.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiment will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 an external view of a display device according to an aspect ofthe present embodiment.

FIG. 2 illustrates an optical system of the display device according toan aspect of the embodiment.

FIG. 3 illustrates a first diffraction element and a second diffractionelement used in a display device according to an aspect of theembodiment.

FIG. 4 illustrates a first diffraction element and a second diffractionelement for spherical waves.

FIG. 5 illustrates a first diffraction element and a second diffractionelement used in a display device according to a first embodiment.

FIG. 6 illustrates a first diffraction element and a second diffractionelement used in a display device according to a first comparativeexample of the embodiment.

FIG. 7 illustrates light beams in a light guide device according to thefirst embodiment and the first comparative example of the embodiment.

FIG. 8 illustrates light beams in a light guide device according to afirst modification of the first embodiment and a first modification ofthe first comparative example of the embodiment.

FIG. 9 illustrates light beams in a light guide device according to asecond modification of the first embodiment and a second modification ofthe first comparative example of the embodiment.

FIG. 10 illustrates a first diffraction element and a second diffractionelement used in a display device according to a second embodiment.

FIG. 11 illustrates a first diffraction element and a second diffractionelement used in a display device according to a second comparativeexample of the embodiment.

FIG. 12 illustrates light beams in the light guide device according tothe second embodiment and the second comparative example of theembodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, embodiments will be described. In the description below,the up-down direction is an X direction, a lateral direction is a Ydirection, and a front-back direction is a Z direction, and X, Y, and Zare used to denote the up-down direction, the lateral direction, and thefront-back direction, respectively.

Example of Display Device Structure

FIG. 1 an external view of a display device 100 according to an aspectof the present embodiment. FIG. 2 illustrates an optical system of thedisplay device 100 according to an aspect of the embodiment. In FIG. 1,the display device 100 is a head-mounted display device and includes aright-eye image light generation device 56 a that emits a laser beam asa light source, a right-eye deflection member 61 a that deflects theimage light emitted from the right-eye image light generation device 56a toward a right eye Ea of an observer M, a left-eye image lightgeneration device 56 b that emits a laser beam as a light source, aleft-eye deflection member 61 b that deflects the image light emittedfrom the left-eye image light generation device 56 b to a left eye Eb ofthe observer M. For example, the display device 100 has a shape similarto the shape of spectacles. Specifically, the display device 100includes a frame 60 that supports the right-eye image light generationdevice 56 a, the right-eye deflection member 61 a, the left-eye imagelight generation device 56 b, and the left-eye deflection member 61 b.The frame 60 is mounted on the head of the observer. The frame 60includes a front section 65 that supports the right-eye deflectionmember 61 a and the left-eye deflection member 61 b. The right-eye imagelight generation device 56 a is disposed on a right temple 62 a and theleft-eye image light generation device 56 b is disposed on a left temple62 b in the frame 60. A first diffraction element 61, which will bedescribed below, is provided in the right-eye deflection member 61 a andthe left-eye deflection member 61 b.

The right-eye image light generation device 56 a and the left-eye imagelight generation device 56 b have similar basic structures, andaccordingly, in FIG. 2, only the structure of the left-eye image lightgeneration device 56 b is described, and the description of theright-eye image light generation device 56 a is omitted. As illustratedin FIG. 2, the left-eye image light generation device 56 b includes alight source section 51 that emits a beam of light for displaying animage, a scanning optical system 20 that has a scanning mirror 21 thatscans the beam of light emitted from the light source section 51 to forman image, and an optical system 52 that outputs a beam of light L0 ascanned by the scanning optical system 20 to the left-eye deflectionmember 61 b. In this aspect, in the optical system 52, a relay lenssystem 54 that includes lenses 541 and 542, and a projection lens system55 are disposed in this order from the scanning optical system 20 towardthe left-eye deflection member 61 b. In the optical system 52, a beamdiameter expanding element 58 is disposed. The beam diameter expandingelement 58 expands the light beam output from the scanning opticalsystem 20 in at least one direction of a first expansion direction B1that corresponds to a first scanning direction A1 (first incidentdirection C1) and a second expansion direction B2 that corresponds to asecond scanning direction A2 (second incident direction C2).

The light source section 51 emits source light that has not opticallybeen modulated or modulated light that has been optically modulated. Inthis aspect, the light source section 51 serves as a modulated lightemitting section that emits optically modulated light. Morespecifically, the light source section 51 includes, as a light source, ared laser device 511(R) that emits red light (R), a green laser device511(G) that emits green light (G), and a blue laser device 511 (B) thatemits blue light (B), and also includes two half mirrors 512 and 513,which combine the optical paths of these laser devices, and a collimatorlens 514. The red laser device 511 (R), the green laser device 511(G),and the blue laser device 511(B) are semiconductor lasers that emitbeams of light that are modulated to have light intensitiescorresponding to respective dots of an image to be displayed undercontrol of a controller (not illustrated).

A scanning optical system 20 scans incident light in the first scanningdirection A1, and the second scanning direction A2 that intersects thefirst scanning direction A1 to generate image light L0 b. Consequently,in this aspect, an image light generation device 70 includes the lightsource section 51 and the scanning optical system 20. The scanningoptical system 20 and the left-eye deflection member 61 b serve as alight guide device 57. The scanning optical system 20 operates under thecontrol of a controller (not illustrated). The scanning optical system20 can be implemented, for example, by a micromirror device manufacturedusing a silicon substrate or the like by microelectromechanical systems(MEMS) technology.

The image light L0 b emitted from the scanning optical system 20 in theimage light generation device 70 is projected toward the left-eyedeflection member 61 b via the relay lens system 54 and the projectionlens system 55. In this aspect, the display device 100 is aretinal-scanning-type projection display device. The image light L0 bemitted by the scanning optical system 20 in the first scanningdirection A1 and the second scanning direction A2, which intersects thefirst scanning direction A1, is deflected by the left-eye deflectionmember 61 b (diffraction element 61) in a first incident direction C1that corresponds to the first scanning direction A1 and a secondincident direction C2 that corresponds to the second scanning directionA2. The image light L0 b reaches a retina E2 through a pupil E1 of aleft eye Eb, and thereby the observer M recognizes the image.

In this aspect, the left-eye deflection member 61 b is provided with thefirst diffraction element 61 that has a reflective volume holographicelement 610. The reflective volume holographic element 610 is apartially reflective diffractive optical element, and the right-eyedeflection member 61 a and the left-eye deflection member 61 b arepartially transmissive reflective combiners. Accordingly, external lightalso enters the left eye Eb via the left-eye deflection member 61 b(combiner), and the user can recognize a superimposed image consistingof the image light L0 a in the display device 100 and the external light(background). The first diffraction element 61 has a concave curvedsurface facing the eye E of the observer, and this structure enablesefficient gathering of the image light L0 a and propagation toward theeye E of the observer.

Structure of the First Diffraction Element 61

FIG. 3 illustrates the first diffraction element 61 and a seconddiffraction element 35 used in the display device 100 according to anaspect of the embodiment. FIG. 4 illustrates the first diffractionelement 61 and the second diffraction element 35 for spherical waves. Inthis aspect, the red light (R), the green light (G), and the blue light(B) enter the first diffraction element 61, and the first diffractionelement 61 diffracts and outputs the light of respective colors inpredetermined directions. Accordingly, as schematically illustrated inFIG. 3, the first diffraction element 61 has first interference fringes611 (R) that have a pitch corresponding to the wavelength of the redlight (R), second interference fringes 611 (G) that have a pitchcorresponding to the wavelength of the green light (G), and thirdinterference fringes 611 (B) that have a pitch corresponding to thewavelength of the blue light (B), the interference fringes being layeredin the thickness direction. These interference fringes 611 (the firstinterference fringes 611 (R), the second interference fringes 611 (G),and the third interference fringes 611 (B)) are recorded in respectivehologram photosensitive layers as variations in refractive indexes,variations in transmittances, and variations in shapes such as theconcave-convex patterns. These interference fringes 611 are inclined inone direction with respect to the incident surface of the firstdiffraction element 61. Accordingly, the first diffraction element 61emits the diffraction light L2 that has the highest diffractionefficiency in one direction when a light beam L1 of a single wavelengthenters from a direction of a normal. The interference fringes 611 can beproduced by interference exposure of the holographic photosensitivelayers to reference light Lr and object light Ls. If the object light Lsis a plane wave, a plurality of interference fringes 611 are produced ina linearly parallel manner. In FIG. 3, the first interference fringes611 (R), the second interference fringes 611 (G), and the thirdinterference fringes 611 (B) are formed in different layers, however, aplurality of types of interference fringes may be formed in a singlelayer. In such a case, the plurality of types of interference fringesare recorded so as to intersect each other.

If the light that enters the first diffraction element 61 is a sphericalwave, to perform interference exposure, for example, a spherical wave isused as the object light Ls. In such a case, a plurality of curvedinterference fringes 611 are produced parallel to each other asschematically illustrated in FIG. 4. If the fringes 611 are inclined inone direction with respect to the incident surface of the firstdiffraction element 61. Accordingly, the first diffraction element 61emits the diffraction light L2 of the spherical wave that has highestdiffraction efficiency in one direction when the light beam L1 of thespherical wave having a single wavelength enters from a direction of anormal. In such a case, the “inclination direction” can be defined as,for example, an inclination of a line T0 that connects both ends T1 andT2 of an interference fringe 611. In the case of the spherical wave,“the incident direction and the output direction” can be defined asdirections extending from the wave source in directions orthogonal tothe plane since the spherical surface becomes substantially equivalentto a plane at a location separated from the wave source.

If the first diffraction element 61 is a reflective diffraction element,the first diffraction element 61 may be a surface-relief diffractionelement (blazed grating) that has a profile of sawtooth grooves, asurface-relief holographic element (blazed holographic grating) formedby combining a holographic element and a surface-relief diffractionelement (blazed holographic grating), or the like. Any of these elementscan output diffracted light of highest diffraction efficiency in onedirection when a light beam enters from a direction of a normal.

Structure of the Second Diffraction Element 35 for Wavelength VariationCorrection

In the display device 100 (the left-eye image light generation device 56b) according to this aspect, when an incident light beam of apredetermined wavelength enters, the first diffraction element 61diffracts the incident light beam and outputs the light in apredetermined direction. However, even if lasers such as the red laserdevice 511 (R), the green laser device 511(G), and the blue laser device511 (B) emit light in a single mode, the light emitted from asemiconductor laser has a certain spectrum width and includes lighthaving wavelengths different from the predetermined wavelength.Furthermore, in some cases, the red laser device 511 (R), the greenlaser device 511(G), and the blue laser device 511 (B) are affected byambient temperatures or other factors and emit light having variouswavelengths. In such a case, among the incident light beams, the firstdiffraction element 61 diffracts the light beams different from thepredetermined wavelength in various diffraction directions (outputdirections), and these variations may result in a decrease in theresolution. To deal with this problem, the display device 100 and thelight guide device 57 according to this aspect are provided with thesecond diffraction element 35 on the optical path from the light sourcesection 51 of the image light generation device 70 to the firstdiffraction element 61. In this aspect, the second diffraction element35 includes a reflective volume holographic element 350.

The red light (R), the green light (G), and the blue light (B) enter thesecond diffraction element 35, and the second diffraction element 35diffracts and outputs the light of respective colors in predetermineddirections. Accordingly, as schematically illustrated in FIG. 3,similarly to the first diffraction element 61, the second diffractionelement 35 has first interference fringes 351 (R) that have a pitchcorresponding to the wavelength of the red light (R), secondinterference fringes 351 (G) that have a pitch corresponding to thewavelength of the green light (G), and third interference fringes 351(B) that have a pitch corresponding to the wavelength of the blue light(B), the interference fringes being layered in the thickness direction.These interference fringes 351 (the first interference fringes 351 (R),the second interference fringes 351 (G), and the third interferencefringes 351 (B)) are recorded in respective hologram photosensitivelayers as variations in refractive indexes, variations intransmittances, and variations in shapes such as the concave-convexpatterns, similarly to the interface fringes 611. These interferencefringes 351 are inclined in one direction with respect to the incidentsurface of the first diffraction element 61. Accordingly, similarly tothe first diffraction element 61, the second diffraction element 35emits diffraction light L2 that has the highest diffraction efficiencyin one direction when a light beam L1 of a single wavelength enters froma direction of a normal. The interference fringes 351 are produced byinterference exposure of the holographic photosensitive layers to thereference light Lr and the object light Ls, similarly to theinterference fringes 611. If the object light Ls is a plane wave, aplurality of interference fringes 351 are produced in a linearlyparallel manner. In the second diffraction element 35, similarly to thefirst diffraction element 61, in some cases, a plurality of interferencefringes are produced in a single layer.

If the light that enters the second diffraction element 35 is aspherical wave, to perform interference exposure, a spherical wave isused as the object light Ls. In such a case, curved interference fringes351 are produced as schematically illustrated in FIG. 4. If theinterference fringes 351 are curved, the interference fringes 351 areinclined in one direction with respect to the incident surface of thesecond diffraction element 35. Accordingly, the second diffractionelement 35 emits the diffraction light L2 of the spherical wave that hashighest diffraction efficiency in one direction when the light beam L1of the spherical wave having the single wavelength enters from thedirection of the normal. In such a case, the “inclination direction” ofthe interference fringe 351 can be defined as, for example, aninclination of a line T0 that connects both ends T1 and T2 of theinterference fringe 351. In the case of the spherical wave, “theincident direction and the output direction” can be defined asdirections extending from the wave source in directions orthogonal to aplane since the spherical surface becomes substantially equivalent to aplane at a location separated from the wave source.

Since the second diffraction element 35 is provided to reduce thevariations in the wavelengths in the first diffraction element 61, thepitches of the interference fringes 351 (the first interference fringes351 (R), the second interference fringes 351 (G), and the thirdinterference fringes 351 (B)) of the second diffraction element 35 arethe same as those of the interference fringes 611 (the firstinterference fringes 611 (R), the second interference fringes 611 (G),and the third interference fringes 611 (B)) of the first diffractionelement 61 respectively. The pitches of the interference fringes 351 and611 are the same in the in-plane direction in the first diffractionelement 61 and the second diffraction element 35. It should be notedthat in the first diffraction element 61, the pitches of theinterference fringes 611 may be different in the in-plane direction. Forexample, in the first diffraction element 61, angles for diffracting theimage light L0 b so as to let the image light L0 b enter the eye of theobserver are different in the central part and in the end parts, andaccordingly, the pitches of the interference fringes 611 may be changeddifferently to correspond to the difference in the angles. In such acase, it is preferable that the pitches of the interference fringes 611in the first diffraction element 61 be within the range between half totwice the interference fringes 351 of the second diffraction element 35.

If the second diffraction element 35 is a reflective diffractionelement, the second diffraction element 35 may be a surface-reliefdiffraction element, a surface-relief holographic element (blazedholographic grating), or the like. Any of these elements can outputdiffraction light of highest diffraction efficiency in one directionwhen a light beam enters from a direction of a normal.

First Embodiment of the Light Guide Device 57

FIG. 5 illustrates the first diffraction element 61 and the seconddiffraction element 35 in the display device 100 according to the firstembodiment. FIG. 6 illustrates the first diffraction element 61 and thesecond diffraction element 35 in the display device 100 according to afirst comparative example of the embodiment. FIG. 7 illustrates lightbeams in the light guide device 57 according to the first embodiment andthe first comparative example of the embodiment. In FIG. 7, the firstdiffraction element 61 is illustrated as a flat-shaped element. In FIGS.5, 6, and 7, light beams of wavelengths optimum for the interferencefringe pitches in the first diffraction element 61 and the seconddiffraction element 35 are indicated by solid lines, and light beams ofwavelengths varied to longer wavelengths than the optimum wavelengthsare indicated by dotted lines. Furthermore, FIGS. 5, 6, and 7schematically illustrate the inclination directions of the interferencefringes 351 and 611.

As illustrated in FIG. 5, in this embodiment, the structures of thefirst diffraction element 61 and the second diffraction element 35 areoptimized depending on the structures of the light guide device 57 andthe like illustrated in FIG. 2. More specifically, in this embodiment,the sum of the number of reflections of the light and the numbers oftimes of intermediate image generation is an even number between thesecond diffraction element 35 and the first diffraction element 61.Accordingly, the second diffraction element 35 and the first diffractionelement 61 are set such that when viewed from direction of normal tovirtual plane that include a direction of a normal to a first incidentplane 615 that is an incident plane of the first diffraction element 61and a direction of a normal to a second incident plane 355 that is anincident plane of the second diffraction element 35, the directions inwhich diffraction light beams that have the highest diffractionefficiencies are output when light enters from the direction of thenormal to the first incident plane 615 and the direction of the normalto the second incident plane 355 are the same side with respect to thedirections of the normals to the incident planes.

More specifically, as illustrated in FIG. 5, according to the firstembodiment, the scanning mirror 21 and an intermediate image generationlens 545 are disposed between the second diffraction element 35 and thefirst diffraction element 61, and between the second diffraction element35 and the first diffraction element 61, the reflection by the scanningmirror 21 and the generation of an intermediate image by theintermediate image generation lens 545 are performed. Accordingly, thesum of the number of reflections and the number of times of generationof intermediate image between the second diffraction element 35 and thefirst diffraction element 61 is two (even number).

Consequently, if a first direction is the direction in which the lighthaving highest diffraction efficiency is output when light enters from adirection of a normal to the first incident plane 615 and a seconddirection is the direction in which the light having highest diffractionefficiency is output when light enters from a direction of a normal tothe second incident plane 355, when viewed from the direction of thenormal to the virtual plane, the direction of the first direction withrespect to the direction of the normal to the first incident plane 615and the direction of the second direction with respect to the directionof the normal to the second incident plane 355 are the same.

More specifically, as indicated by the alternate long and short dashedline L11 in FIG. 5, when a light beam enters from a direction of anormal to the first incident plane 615 of the first diffraction element61, the first direction in which diffraction light (alternate long andshort dashed line L12) that has highest diffraction efficiency is outputis on a clockwise CW side with respect to the direction of the normal tothe first incident plane 615. As indicated by the alternate long andshort dashed line L21 in FIG. 5, when a light beam enters from adirection of a normal to the second incident plane 355 of the seconddiffraction element 35, the second direction in which diffraction light(alternate long and short dashed line L22) that has highest diffractionefficiency is output is on the clockwise CW side with respect to thedirection of the normal to the second incident plane 355, the same sideas in the first diffraction element 61. Such a structure is implementedby providing the interference fringes 611 and the interference fringes351 such that the inclination direction of the interference fringes 611and the inclination direction of the interference fringes 351 are thesame, the inclination directions have been described with reference toFIG. 3.

In this structure, as illustrated in FIG. 5 and FIG. 7(a), with respectto a case where a light beam (solid line L1) of an optimum wavelengthenters from a direction of a normal to the second incident plane 355 ofthe second diffraction element 35, a diffraction light (dotted line L2)produced when a light beam of a wavelength longer than the optimumwavelength enters is inclined toward the clockwise CW side. Accordingly,when light enters the first incident plane 615 of the first diffractionelement 61 via the scanning mirror 21 and the intermediate imagegeneration lens 545, the diffraction light of the light beam that hasthe wavelength longer than the optimum wavelength enters from the sideinclined toward the clockwise CW side compared to the light beam of theoptimum wavelength. Consequently, the light beam of the optimumwavelength and the light beam of the wavelength longer than the optimumwavelength are output toward the same direction from the firstdiffraction element 61, and thereby the decrease in resolution can bereduced. According to the embodiment, for example, image displacementdue to variations in the wavelengths can be reduced to about one pixelor less.

In contrast, if a structure according to a first comparative exampleillustrated in FIG. 6 is used when the sum of the number of reflectionsand the number of times of intermediate image generation between thesecond diffraction element 35 and the first diffraction element 61 istwo (even number), the light beam of an optimum wavelength and the lightbeam of a wavelength longer than the optimum wavelength are outputtoward different directions from the first diffraction element 61, andthereby the resolution is largely decreased. That is, in the firstcomparative example illustrated in FIG. 6, as indicated by the alternatelong and short dashed line L11 in FIG. 6, when a light beam enters froma direction of a normal to the first incident plane 615 of the firstdiffraction element 61, the first direction in which diffraction light(alternate long and short dashed line L12) that has highest diffractionefficiency is output is on the clockwise CW side with respect to thedirection of the normal to the first incident plane 615. In contrast, asindicated by the alternate long and short dashed line L21 in FIG. 6,when a light beam enters from a direction of a normal to the seconddiffraction element 35, a location where diffraction light (alternatelong and short dashed line L22) that has highest diffraction efficiencyis output is on the counterclockwise CCW side with respect to thedirection of the normal to the second incident plane 355, the oppositeside as in the first diffraction element 61.

In this structure, as illustrated in FIG. 6 and FIG. 7(c), with respectto a case where a light beam (solid line L1) of an optimum wavelengthenters from a direction of a normal to the second incident plane 355 ofthe second diffraction element 35, a diffraction light (dotted line L2)produced when a light beam of a wavelength longer than the optimumwavelength enters is inclined toward the counterclockwise CCW side.Accordingly, when the diffraction light of the light beam of thewavelength longer than the optimum wavelength enters the first incidentplane 615 of the first diffraction element 61 via the scanning mirror 21and the intermediate image generation lens 545, the diffraction lightenters from the side inclined toward the counterclockwise CCW sidecompared to the light beam of the optimum wavelength. Consequently, thelight beam of the optimum wavelength and the light beam of thewavelength longer than the optimum wavelength are output toward thedifferent directions from the first diffraction element 61, and therebythe resolution can be largely decreased. Accordingly, for example, imagedisplacement of about 10 pixels may occur due to variations in thewavelengths.

FIG. 7(b) illustrates a light beam that enters from an oblique directiontoward the second incident plane 355 of the second diffraction element35 according to the first embodiment. In this case, similarly to thecase described with reference to FIG. 5, with respect to a case where abeam of light (solid line L1) of an optimum wavelength enters, adiffraction light (dotted line L2) produced when a light beam of awavelength longer than the optimum wavelength enters is inclined towardthe clockwise CW side. Accordingly, when light enters the first incidentplane 615 of the first diffraction element 61 via the scanning mirror 21and the intermediate image generation lens 545, the diffraction light ofthe light beam that has the wavelength longer than the optimumwavelength enters from the side inclined toward the clockwise CW sidecompared to the light beam of the optimum wavelength. Consequently, thelight beam of the optimum wavelength and the light beam of thewavelength longer than the optimum wavelength are output toward the samedirection from the first diffraction element 61, and thereby thedecrease in resolution can be reduced.

FIG. 7(d) illustrates a light beam that enters from an oblique directiontoward the second incident plane 355 of the second diffraction element35 according to the first comparative example. In this case, similarlyto the case described with reference to FIG. 6, with respect to a casewhere a beam of light (solid line L1) of an optimum wavelength enters, adiffraction light (dotted line L2) produced when a light beam of awavelength longer than the optimum wavelength enters is inclined towardthe counterclockwise CCW side. Accordingly, the diffraction light of thelight beam of the wavelength longer than the optimum wavelength entersfrom the side inclined toward the counterclockwise CCW side compared tothe light beam of the optimum wavelength, when the diffraction lightenters the first incident plane 615 of the first diffraction element 61via the scanning mirror 21 and the intermediate image generation lens545. Consequently, the light beam of the optimum wavelength and thelight beam of the wavelength longer than the optimum wavelength areoutput toward the different directions from the first diffractionelement 61, and thereby the resolution may be largely decreased.

First Modification of the First Embodiment

FIG. 8 illustrates light beams in the light guide device 57 according toa first modification of the first embodiment and a first modification ofthe first comparative example of the embodiment. In FIG. 8, the firstdiffraction element 61 is illustrated as a flat-shaped element. In FIG.8, light beams of wavelengths optimum for the interference fringepitches in the first diffraction element 61 and the second diffractionelement 35 are indicated by solid lines, and light beams of wavelengthsvaried to longer wavelengths than the optimum wavelengths are indicatedby dotted lines. Furthermore, FIG. 8 schematically illustrates theinclination directions of the interference fringes 351 and 611.

FIG. 8 illustrates light beams in the light guide device 57 according tothe first modification of the first embodiment and the firstmodification of the first comparative example of the embodiment when thesum of the number of reflections and the number of times of intermediateimage generation between the second diffraction element 35 and the firstdiffraction element 61 is two (even number). That is, in the firstmodification of the first embodiment, the intermediate image generationlens 545 is not provided, and between the second diffraction element 35and the first diffraction element 61, the reflection by the scanningmirror 21 and the reflection by a mirror 546 are performed. Accordingly,the sum of the number of reflections and the number of times ofintermediate image generation between the second diffraction element 35and the first diffraction element 61 is two (even number).

In this structure, according to the first modification of the firstembodiment, as illustrated in FIG. 8(a), with respect to a case wherethe light beam (solid line L1) of an optimum wavelength enters from adirection of a normal to the second incident plane 355 of the seconddiffraction element 35, a diffraction light (dotted line L2) producedwhen a light beam of a wavelength longer than the optimum wavelengthenters is inclined toward the clockwise CW side. Accordingly, when lightenters the first incident plane 615 of the first diffraction element 61via the scanning mirror 21 and the mirror 546, the diffraction light ofthe light beam that has the wavelength longer than the optimumwavelength enters from the side inclined toward the clockwise CW sidecompared to the light beam of the optimum wavelength. Consequently, thelight beam of the optimum wavelength and the light beam of thewavelength longer than the optimum wavelength are output toward the samedirection from the first diffraction element 61, and thereby thedecrease in resolution can be reduced.

According to the first modification of the first embodiment, asillustrated in FIG. 8(b), with respect to a case where the light beam(solid line L1) of an optimum wavelength enters from an obliquedirection to the second incident plane 355 of the second diffractionelement 35, a diffraction light (dotted line L2) produced when a lightbeam of a wavelength longer than the optimum wavelength enters isinclined toward the clockwise CW side. Accordingly, when light entersthe first incident plane 615 of the first diffraction element 61 via thescanning mirror 21 and the mirror 546, the diffraction light of thelight beam that has the wavelength longer than the optimum wavelengthenters from the side inclined toward the clockwise CW side compared tothe light beam of the optimum wavelength. Consequently, the light beamof the optimum wavelength and the light beam of the wavelength longerthan the optimum wavelength are output toward the same direction fromthe first diffraction element 61, and thereby the decrease in resolutioncan be reduced.

In contrast, according to the first modification of the firstcomparative example, as illustrated in FIG. 8(c), with respect to a casewhere the light beam (solid line L1) of the optimum wavelength entersfrom a direction of a normal to the second incident plane 355 of thesecond diffraction element 35, a diffraction light (dotted line L2)produced when a light beam of a wavelength longer than the optimumwavelength enters is inclined toward the counterclockwise CCW side.Accordingly, when light enters the first incident plane 615 of the firstdiffraction element 61 via the scanning mirror 21 and the mirror 546,the diffraction light of the light beam that has the wavelength longerthan the optimum wavelength enters from the side inclined toward thecounterclockwise CCW side compared to the light beam of the optimumwavelength. Consequently, the light beam of the optimum wavelength andthe light beam of the wavelength longer than the optimum wavelength areoutput toward the different directions from the first diffractionelement 61, and thereby the resolution may be largely decreased.

According to the first modification of the first comparative example, asillustrated in FIG. 8(d), with respect to a case where a light beam(solid line L1) of an optimum wavelength enters the second incidentplane 355 of the second diffraction element 35 from an obliquedirection, a diffraction light (dotted line L2) produced when a lightbeam of a wavelength longer than the optimum wavelength enters isinclined toward the counterclockwise CCW side. Accordingly, when lightenters the first incident plane 615 of the first diffraction element 61via the scanning mirror 21 and the mirror 546, the diffraction light ofthe light beam that has the wavelength longer than the optimumwavelength enters from the side inclined toward the counterclockwise CCWside compared to the light beam of the optimum wavelength. Consequently,the light beam of the optimum wavelength and the light beam of thewavelength longer than the optimum wavelength are output toward thedifferent directions from the first diffraction element 61, and therebythe resolution may be largely decreased.

Second Modification of the First Embodiment

FIG. 9 illustrates light beams in the light guide device 57 according toa second modification of the first embodiment and a second modificationof the first comparative example of the embodiment. In FIG. 9, the firstdiffraction element 61 is illustrated as a flat-shaped element. In FIG.9, light beams of wavelengths optimum for the interference fringepitches in the first diffraction element 61 and the second diffractionelement 35 are indicated by solid lines, and light beams of wavelengthsvaried to longer wavelengths than the optimum wavelengths are indicatedby dotted lines. Furthermore, FIG. 9 schematically illustrates theinclination directions of the interference fringes 351 and 611.

FIG. 9 illustrates light beams in the light guide device 57 according tothe second modification of the first embodiment and the secondmodification of the first comparative example of the embodiment when thesum of the number of reflections and the number of times of intermediateimage generation between the second diffraction element 35 and the firstdiffraction element 61 is zero (even number). That is, in the secondmodification of the first embodiment and the second modification of thefirst comparative example of the embodiment, between the seconddiffraction element 35 and the first diffraction element 61, thescanning mirror 21 and the intermediate image generation lens 545 arenot provided. Accordingly, the sum of the number of reflections and thenumber of times of intermediate image generation between the seconddiffraction element 35 and the first diffraction element 61 is zero(even number).

In this structure, according to the second modification of the firstembodiment, as illustrated in FIG. 9(a), with respect to a case where alight beam (solid line L1) of an optimum wavelength enters from adirection of a normal to the second incident plane 355 of the seconddiffraction element 35, a diffraction light (dotted line L2) producedwhen a light beam of a wavelength longer than the optimum wavelengthenters inclines toward the clockwise CW side. Accordingly, when lightenters the first incident plane 615 of the first diffraction element 61,the diffraction light of the light beam that has the wavelength longerthan the optimum wavelength enters from the side inclined toward theclockwise CW side compared to the light beam of the optimum wavelength.Consequently, the light beam of the optimum wavelength and the lightbeam of the wavelength longer than the optimum wavelength are outputtoward the same direction from the first diffraction element 61, andthereby the decrease in resolution can be reduced.

According to the second modification of the first embodiment, asillustrated in FIG. 9(b), with respect to a case where a light beam(solid line L1) of an optimum wavelength enters the second incidentplane 355 of the second diffraction element 35 from an obliquedirection, a diffraction light (dotted line L2) produced when a lightbeam of a wavelength longer than the optimum wavelength enters isinclined toward the clockwise CW side.

Accordingly, when light enters the first incident plane 615 of the firstdiffraction element 61, the diffraction light of the light beam that hasthe wavelength longer than the optimum wavelength enters from the sideinclined toward the clockwise CW side compared to the light beam of theoptimum wavelength. Consequently, the light beam of the optimumwavelength and the light beam of the wavelength longer than the optimumwavelength are output toward the same direction from the firstdiffraction element 61, and thereby the decrease in resolution can bereduced.

In contrast, according to the second modification of the firstcomparative example, as illustrated in FIG. 9(c), with respect to a casewhere a light beam (solid line L1) of an optimum wavelength enters froma direction of a normal to the second incident plane 355 of the seconddiffraction element 35, a diffraction light (dotted line L2) producedwhen a light beam of a wavelength longer than the optimum wavelengthenters is inclined toward the counterclockwise CCW side. Accordingly,when light enters the first incident plane 615 of the first diffractionelement 61, the diffraction light of the light beam that has thewavelength longer than the optimum wavelength enters from the sideinclined toward the counterclockwise CCW side compared to the light beamof the optimum wavelength. Consequently, the light beam of the optimumwavelength and the light beam of the wavelength longer than the optimumwavelength are output toward the different directions from the firstdiffraction element 61, and thereby the resolution may be largelydecreased.

According to the second modification of the first comparative example,as illustrated in FIG. 9(d), with respect a case where a light beam(solid line L1) of an optimum wavelength enters the second incidentplane 355 of the second diffraction element 35 from an obliquedirection, a diffraction light (dotted line L2) produced when a lightbeam of a wavelength longer than the optimum wavelength enters isinclined toward the counterclockwise CCW side. Accordingly, when lightenters the first incident plane 615 of the first diffraction element 61,the diffraction light of the light beam that has the wavelength longerthan the optimum wavelength enters from the side inclined toward thecounterclockwise CCW side compared to the light beam of the optimumwavelength. Consequently, the light beam of the optimum wavelength andthe light beam of the wavelength longer than the optimum wavelength areoutput toward the different directions from the first diffractionelement 61, and thereby the resolution may be largely decreased.

Second Embodiment of the Light Guide Device 57

FIG. 10 illustrates the first diffraction element 61 and the seconddiffraction element 35 in the display device 100 according to a secondembodiment. FIG. 11 illustrates the first diffraction element 61 and thesecond diffraction element 35 in the display device 100 according to asecond comparative example of the embodiment. FIG. 12 illustrates lightbeams in the light guide device 57 according to the second embodimentand the second comparative example of the embodiment. In FIG. 12, thefirst diffraction element 61 is illustrated as a flat-shaped element. InFIG. 11, the mirror 547 and the intermediate image generation lens 545are omitted so as to simplify the sum of the number of reflections andthe number of the times of intermediate image generation to one. InFIGS. 10, 11, and 12, light beams of wavelengths optimum for theinterference fringe pitches in the first diffraction element 61 and thesecond diffraction element 35 are indicated by solid lines, and lightbeams of wavelengths varied to longer wavelengths than the optimumwavelengths are indicated by dotted lines. Furthermore, FIGS. 10, 11,and 12 schematically illustrate the inclination directions of theinterference fringes 351 and 611.

As illustrated in FIG. 10, in the second embodiment and the secondcomparative example, similarly to the first embodiment, the structuresof the first diffraction element 61 and the second diffraction element35 are optimized depending on the structure of the light guide device 57and the like illustrated in FIG. 2. More specifically, in the secondembodiment and the second comparative example, the sum of the number ofreflections of the light and the number of times of intermediate imagegeneration between the second diffraction element 35 and the firstdiffraction element 61 is an odd number. Accordingly, the seconddiffraction element 35 and the first diffraction element 61 are set suchthat when viewed from direction of normal to virtual plane that includea direction of a normal to the first incident plane 615 of the firstdiffraction element 61 and a direction of a normal to the secondincident plane 355 of the second diffraction element 35, the directionsin which diffraction light beams that have the highest diffractionefficiencies are output when light enters from the direction of thenormal to the first incident plane 615 and the direction of the normalto the second incident plane 355 are the opposite sides with respect tothe directions of the normals to the incident planes.

More specifically, as illustrated in FIG. 10, according to the secondembodiment, a mirror 547, the scanning mirror 21, and the intermediateimage generation lens 545 are disposed between the second diffractionelement 35 and the first diffraction element 61, and between the seconddiffraction element 35 and the first diffraction element 61, thereflection by the mirror 547, the reflection by the scanning mirror 21,and the generation of an intermediate image by the intermediate imagegeneration lens 545 are performed. Accordingly, the sum of the number ofreflections and the number of times of intermediate image generationbetween the second diffraction element 35 and the first diffractionelement 61 is three (odd number).

Consequently, if a first direction is the direction in which the lighthaving highest diffraction efficiency is output when light enters from adirection of a normal to the first incident plane 615 and a seconddirection is the direction in which the light having highest diffractionefficiency is output when light enters from a direction of a normal tothe second incident plane 355, when viewed from the direction of thenormal to the virtual plane, the direction of the first direction withrespect to the direction of the normal to the first incident plane 615and the direction of the second direction with respect to the directionof the normal to the second incident plane 355 are opposite to eachother.

For example, as indicated by the alternate long and short dashed lineL11 in FIG. 10, when a light beam enters from a direction of a normal tothe first incident plane 615 of the first diffraction element 61, thefirst direction in which diffraction light (alternate long and shortdashed line L12) that has highest diffraction efficiency is output is onthe clockwise CW side with respect to the direction of the normal to thefirst incident plane 615. Furthermore, as indicated by the alternatelong and short dashed line L21 in FIG. 10, when a light beam enters froma direction of a normal to the second incident plane 355 of the seconddiffraction element 35, the second direction in which diffraction light(alternate long and short dashed line L22) that has highest diffractionefficiency is output is on the counterclockwise CCW side with respect tothe direction of the normal to the second incident plane 355, theopposite side as in the first diffraction element 61. Such a structureis implemented by providing the interference fringes 611 and theinterference fringes 351 such that the inclination direction of theinterference fringes 611 and the inclination direction of theinterference fringes 351 are opposite each other, the inclinationdirections have been described with reference to FIG. 3.

In this structure, as illustrated in FIG. 10 and FIG. 12(a), withrespect to a case where a light beam (solid line L1) of an optimumwavelength enters from a direction of a normal to the second incidentplane 355 of the second diffraction element 35, a diffraction light(dotted line L2) produced when a light beam of a wavelength longer thanthe optimum wavelength enters is inclined toward the counterclockwiseCCW side. Accordingly, when light enters the first incident plane 615 ofthe first diffraction element 61 via the mirror 547, the scanning mirror21, and the intermediate image generation lens 545, the diffractionlight of the light beam that has the wavelength longer than the optimumwavelength enters from the side inclined toward the clockwise CW sidecompared to the light beam of the optimum wavelength. Consequently, thelight beam of the optimum wavelength and the light beam of thewavelength longer than the optimum wavelength are output toward the samedirection from the first diffraction element 61, and thereby thedecrease in resolution can be reduced.

In contrast, if a structure according to the second comparative exampleillustrated in FIG. 11 is used when the sum of the number of reflectionsand the number of times of intermediate image generation between thesecond diffraction element 35 and the first diffraction element 61 isthree (odd number), the light beam of an optimum wavelength and thelight beam of a wavelength longer than the optimum wavelength are outputtoward different directions from the first diffraction element 61, andthereby the resolution may largely decreased. That is, in the secondcomparative example illustrated in FIG. 11, as indicated by thealternate long and short dashed line L11 in FIG. 11, when a light beamenters from a direction of a normal to the first incident plane 615 ofthe first diffraction element 61, the first direction in whichdiffraction light (alternate long and short dashed line L12) that hashighest diffraction efficiency is output is on the clockwise CW sidewith respect to the direction of the normal to the first incident plane615. Furthermore, as indicated by the alternate long and short dashedline L21 in FIG. 11, when a light beam enters from a direction of anormal to the second incident plane 355 of the second diffractionelement 35, the location toward which diffraction light (alternate longand short dashed line L22) that has highest diffraction efficiency isoutput is on the clockwise CW side with respect to the direction of thenormal to the second incident plane 355, the same side as in the firstdiffraction element 61.

In this structure, as illustrated in FIG. 11 and FIG. 12(c), withrespect to a case where a light beam (solid line L1) of an optimumwavelength enters from a direction of a normal to the second incidentplane 355 of the second diffraction element 35, a diffraction light(dotted line L2) produced when a light beam of a wavelength longer thanthe optimum wavelength enters is inclined toward the clockwise CW side.Accordingly, when light enters the first incident plane 615 of the firstdiffraction element 61 via the mirror 547, the scanning mirror 21, andthe intermediate image generation lens 545, the diffraction light of thelight beam that has the wavelength longer than the optimum wavelengthenters from the side inclined toward the counterclockwise CCW sidecompared to the light beam of the optimum wavelength. Consequently, thelight beam of the optimum wavelength and the light beam of thewavelength longer than the optimum wavelength are output toward thedifferent directions from the first diffraction element 61, and therebythe resolution may be largely decreased.

FIG. 12(b) illustrates a light beam that enters the second incidentplane 355 of the second diffraction element 35 from an oblique directionaccording to the second embodiment. In this case, similarly to the casedescribed with reference to FIG. 10, with respect to a case where a beamof light (solid line L1) of an optimum wavelength enters, a diffractionlight (dotted line L2) produced when a light beam of a wavelength longerthan the optimum wavelength enters is inclined toward thecounterclockwise CCW side. Accordingly, when light enters the firstincident plane 615 of the first diffraction element 61 via the mirror547, the scanning mirror 21, and the intermediate image generation lens545, the diffraction light of the light beam that has the wavelengthlonger than the optimum wavelength enters from the side inclined towardthe clockwise CW side compared to the light beam of the optimumwavelength. Consequently, the light beam of the optimum wavelength andthe light beam of the wavelength longer than the optimum wavelength areoutput toward the same direction from the first diffraction element 61,and thereby the decrease in resolution can be reduced.

FIG. 12(d) illustrates a light beam that enters the second incidentplane 355 of the second diffraction element 35 from an oblique directionaccording to the second comparative example. In this case, similarly tothe case described with reference to FIG. 11, with respect to a casewhere a beam of light (solid line L1) of an optimum wavelength enters, adiffraction light (dotted line L2) produced when a light beam of awavelength longer than the optimum wavelength enters is inclined towardthe clockwise CW side. Accordingly, when light enters the first incidentplane 615 of the first diffraction element 61 via the mirror 547, thescanning mirror 21, and the intermediate image generation lens 545, thediffraction light of the light beam that has the wavelength longer thanthe optimum wavelength enters from the side inclined toward thecounterclockwise CCW side compared to the light beam of the optimumwavelength. Consequently, the light beam of the optimum wavelength andthe light beam of the wavelength longer than the optimum wavelength areoutput toward the different directions from the first diffractionelement 61, and thereby the resolution may be largely decreased.

Other Embodiments

Although the image light generation device 70 includes the light sourcesection 51 and the scanning optical system 20 in the above-describedembodiments, according to another embodiment, a display device thatgenerates image light with a display panel such as a liquid crystalpanel, an organic electroluminescence display panel, or a display panelthat uses a micromirror may be employed. In the above-describedembodiments, a transmissive volume holographic element or a blazeddiffraction element may be used as one of or both of the firstdiffraction element 61 and the second diffraction element 35.

The entire disclosure of Japanese Patent Application No. 2016-049266,filed Mar. 14, 2016 is expressly incorporated by reference herein.

What is claimed is:
 1. A display device comprising: an image lightgeneration device configured to output an image light; a firstdiffraction element configured to diffract the image light that hasentered a first incident plane such that the image light is directed toan eye of an observer; and a second diffraction element disposed on anoptical path between the image light generation device and the firstdiffraction element, the second diffraction element being configured todiffract the image light that has entered a second incident plane suchthat the image light is directed to the first diffraction element,wherein the first diffraction element has highest diffraction efficiencyin a first direction when light enters from a direction of a normal tothe first incident plane, the second diffraction element has highestdiffraction efficiency in a second direction when light enters from adirection of a normal to the second incident plane, the firstdiffraction element and the second diffraction element are disposed suchthat, if the sum of the number of reflections of light and the number oftimes of intermediate image generation between the second diffractionelement and the first diffraction element is an even number, when viewedfrom direction of normal to virtual plane that include a normal to thefirst incident plane and a normal to the second incident plane, thedirection of the first direction with respect to the direction of thenormal to the first incident plane and the direction of the seconddirection with respect to the direction of the normal to the secondincident plane are the same as each other, and the first diffractionelement and the second diffraction element are disposed such that, ifthe sum of the number of reflections of light and the number of times ofintermediate image generation between the second diffraction element andthe first diffraction element is an odd number, when viewed from thedirection of the normal to the virtual plane that include the normal tothe first incident plane and the normal to the second incident plane,the direction of the first direction with respect to the direction ofthe normal to the first incident plane and the direction of the seconddirection with respect to the direction of the normal to the secondincident plane are different from each other.
 2. The display deviceaccording to claim 1, wherein the first diffraction element and thesecond diffraction element are reflective holographic elements.
 3. Thedisplay device according to claim 2, wherein the first diffractionelement and the second diffraction element are reflective volumeholographic elements.
 4. The display device according to claim 2,wherein the first diffraction element and the second diffraction elementhave a plurality of interference fringes linearly extending parallel toeach other.
 5. The display device according to claim 2, wherein thefirst diffraction element and the second diffraction element have aplurality of curved interference fringes extending parallel to eachother.
 6. The display device according to claim 2, wherein the firstdiffraction element and the second diffraction element have a pluralityof kinds of interference fringes of different pitches.
 7. A light guidedevice comprising: a first diffraction element configured to diffract alight that has been emitted from a light source and that has entered afirst incident plane; and a second diffraction element disposed on anoptical path between the light source and the first diffraction element,the second diffraction element being configured to diffract the lightthat has entered a second incident plane such that the image light isdirected to the first diffraction element, wherein the first diffractionelement has highest diffraction efficiency in a first direction whenlight enters from a direction of a normal to the first incident plane,the second diffraction element has highest diffraction efficiency in asecond direction when light enters from a direction of a normal to thesecond incident plane, the first diffraction element and the seconddiffraction element are disposed such that, if the sum of the number ofreflections of light and the number of times of intermediate imagegeneration between the second diffraction element and the firstdiffraction element is an even number, when viewed from direction ofnormal to virtual plane that include a normal to the first incidentplane and a normal to the second incident plane, the direction of thefirst direction with respect to the direction of the normal to the firstincident plane and the direction of the second direction with respect tothe direction of the normal to the second incident plane are the same aseach other, and the first diffraction element and the second diffractionelement are disposed such that, if the sum of the number of reflectionsof light and the number of times of intermediate image generationbetween the second diffraction element and the first diffraction elementis an odd number, when viewed from the direction of the normal to thevirtual plane that include the normal to the first incident plane andthe normal to the second incident plane, the direction of the firstdirection with respect to the direction of the normal to the firstincident plane and the direction of the second direction with respect tothe direction of the normal to the second incident plane are differentfrom each other.
 8. The light guide device according to claim 7, whereinthe first diffraction element and the second diffraction element arereflective holographic elements.
 9. The light guide device according toclaim 8, wherein the first diffraction element and the seconddiffraction element are reflective volume holographic elements.